At the outbreak of the Mexican-American War, McCulloch raised a command of Texas Rangers and eventually become Zachary Taylor’s chief of scouts. McCulloch's Rangers soon gained fame throughout the U.S. for their daring reconnaissance expeditions into Northern Mexico.
Samuel Reid, a volunteer from Louisiana, offers the following description of McCulloch and his Ranger company:
men in groups with long beards and mustaches, dressed in every variety of garment, with one exception, the slouched hat, the unmistakable uniform of a Texas ranger, and a brace of pistols around their waists, [who] were occupied drying their blankets, cleaning and fixing their guns, and some employed cooking at different fires, while others were grooming their horses. A rougher-looking set we never saw. They were without tents, and a miserable shed afforded them the only shelter. Captain McCulloch introduced us to his officers and many of his men, who appeared orderly and well-mannered people. But from their rough exterior, it was hard to tell who or what they were. Notwithstanding their ferocious and outlaw look, there were among them doctors and lawyers and many a college graduate.